Holographic memory or storage systems involve the three-dimensional storage of holographic representations of data elements (i.e., holograms) as a pattern of varying refractive index and/or absorption imprinted in the volume of a storage medium such as a crystal of lithium niobate. Holographic memory systems (HMS) are characterized by their high density storage potential and the potential speed with which the stored data is randomly accessed and transferred.
In general, holographic memory systems operate by combining a data encoded object beam with a phase coherent reference beam to create an interference pattern throughout a photosensitive storage medium such as a holographic memory cell (HMC). The interference pattern induces material alterations in the HMC that record a hologram. The response of the hologram in the storage medium is a function of the relative amplitudes and polarization states of, and phase differences between, the object beam and the reference beam. It is also highly dependent on the incident beam's wavelengths and angles at which the object beam and the reference beam are projected into the storage medium.
Holographically stored data is reconstructed by projecting a reference beam similar to the reference beam used in storing the data into the HMC at the same angle, wavelength, phase and position used to produce the hologram. The hologram and the reference beam interact to reconstruct the stored object beam (i.e., the data). The reconstructed object beam may then be detected, e.g., using a photodetector array. The recovered data may then be post-processed for delivery to output devices.
Typically, the dynamic range of the holographic storage medium is larger than what is needed to store a single hologram with an acceptable signal-to-noise ratio. Therefore, it is desirable to multiplex a number of holograms at one location in the storage medium to achieve greater storage density. One multiplexing technique is phase correlation multiplexing (PCM), in which correlation selectivity and Bragg selectivity are used for differentiating overlapping holograms within a storage medium. Correlation selectivity relies on the differences in amplitude, phase and angle content of the reference beam produced by the relative shift (in any direction) of the storage medium with respect to its reference beam.
However, multiplexing schemes such as PCM require relatively complex reference beams whose formation involve complicated phase masks, high quality lenses, and Fourier plane spatial filtering. Unfortunately, the phase masks are delicate in structure, the lenses are expensive and bulky, and the necessary Fourier plane spatial filters block much of the incoming optical energy, greatly increasing the system's power budget. Also, for PCM holographic memory systems, the alignment of these elements is critical down to the micron (.mu.m) level and typically needs to be consistent from system to system. The level of such consistency is often difficult if not impossible to achieve using conventional components and techniques. A holographic optical element (HOE) can be used to generate or reconstruct a reference beam for the holographic memory system. The HOE thus provides a relatively inexpensive, simple and reproducibly consistent replacement for one or more of the optical elements typically required to reproduce a reference beam in a HMS. An exemplary HOE is disclosed in application Ser. No. 08/968,024, the entire disclosure of which is incorporated herein by reference.
When using PCM or other correlation selectivity techniques within holographic storage systems, the resulting information stored in the HMC is characterized by relatively high resolution and thus high selectivity. While high resolution and high selectivity are desirable, and even necessary, for high-density recording of holograms in the HMC, retrieving restored holographic data also requires high resolution and high selectivity techniques and devices.
Therefore, it is desirable to increase the width of the selectivity function of the stored hologram to thereby provide the ability to search for and retrieve stored holographic data more quickly.